Aircraft are required to ground taxi between locations on airfields. An example is taxiing between a runway and the location (e.g. terminal gate) at which the aircraft's passengers are to board or disembark. Typically, such taxiing is achieved by using the thrust from the aircraft's engines to propel the aircraft forwards so that the landing gear wheels are caused to rotate. Since ground taxi speeds are necessarily relatively low, the engines must be run at a very low power. This means that there is a relatively high fuel consumption as a result of the poor propulsion efficiency at this low power. This leads to an increased level of both atmospheric and noise pollution locally around airports. Moreover, even when the engines are run at low power it is generally necessary to apply the wheel brakes to limit ground taxi speeds, leading to a high degree of brake wear.
Reversing of a civil aircraft, e.g. away from a terminal gate, using its main engines is not permitted. When reversing is necessary, or in other situations where ground taxiing via main engine thrust is not practicable, tow trucks are used to manoeuvre aircraft around. This process is laborious and costly.
Several autonomous ground taxi systems for driving the wheels while the aircraft is on the ground have been proposed in recent years. Since brakes are installed within the wheels of aircraft main landing gear, this region is very congested and there is little room for installing drive motors. Development of autonomous taxi systems has therefore typically been limited to the nose landing gear. An example is US2006/0065779, which discloses a powered nose aircraft wheel system in which a clutch is used to switch between a mode in which the wheel can spin freely and a mode in which the wheel can be driven by an electric motor. The clutch can also operate to enable the motor to pre-spin the wheel prior to landing. The clutch comprises an automatically disengaged cone clutch mechanism.
However, nose landing gears only support a small fraction of the vertical loads supported by the landing gear as a whole during ground taxi operations (approximately 5% of the aircraft weight). The present inventors consider that there will therefore be insufficient traction between a driven nose landing gear wheel and the ground. This is a particular concern when the aircraft centre of gravity is towards its aft limit and when the ground surface is slippery.